Photo: save path, is-local, content bytes, metadata and EXIF getters (closes #141)

`Photo` gains:

- `savePath` and `isLocal`: synchronous, disk only. Where `lib.backup()` writes the original under the library's `downloadDirectory`, and whether all of it is there; a copy only in the cache does not count.
- `content()` and `exif()`: async, may download. `exif()` reads the common EXIF fields of a JPEG and returns `{}` for anything else.
- The getters `modifiedAt` and `hash`, also on `PhotoRecord`, and `year`.

For a live photo the backup has not stored yet, `savePath` carries the title's extension, and the image may be stored under a different one. The JPEG EXIF scan moved to `src/exif.ts`. The exported `PhotoContent` interface gains `savePath` and `isLocal`.

Judgement call: `iso` is read only when the file stores it as a single number.

Model: opus-5-5

test/cli/metadata-exif.test.ts

@@ -8,10 +8,8 @@
  */
 
 import { describe, expect, it } from "vitest";
-import {
-    extractExifFromJpeg,
-    extractImageMetadata,
-} from "../../src/metadata-backup.js";
+import { extractExifFromJpeg } from "../../src/exif.js";
+import { extractImageMetadata } from "../../src/metadata-backup.js";
 
 const SOI = [0xff, 0xd8]; // start of image
 const SOS = [0xff, 0xda, 0x00, 0x02]; // start of scan, where the scan stops